1. Field of the Invention
This invention concerns a plastic packaged device with a hollow portion enclosing a chip element, e.g., a semiconductor chip and a surface acoustic wave device, and concerns a method of manufacturing the same.
2. Description of the Prior Art
In the case where a surface acoustic wave device is packaged with a plastic material, it is necessary to form a hollow portion over the chip to allow for vibration of the surface acoustic wave device
Conventionally, a construction of a plastic packaged device with a hollow portion and a method of manufacturing the same is known (see: Japanese laid open patent application No:60-53058).
FIGS. 1 and 2 show generally the construction and the method of the prior art. In the prior art, a blowing agent 14 diluted with ethanol is spread over a chip 10 mounted on a die pad portion 11 of a lead frame. The chip 10 is electrically connected to inner lead portions 12 with bonding wires 13. Next, the ethanol is volatilized, and a plastic packaging process is carried out. In this packaging process, the blowing agent 14 is blown off, and a hollow portion 27 is formed over the chip 10 (FIG. 2).
In the prior art, the die pad portion 11 is flat. Thus, to form a dome-shaped hollow portion, the blowing agent 14 is spread in a dome shape using the surface tension of the blowing agent. By expanding the blowing agent by application of a specific temperature during the packaging process, the dome-shaped hollow portion 27 is formed (FIG. 2).
However, in the case where the ratio of the area of the chip 10 to the die pad portion is high, it is difficult to cover the chip with the blowing agent with a sufficient thickness over the chip. Particularly, at the edge portions of the chip 10, the hollow portion becomes very narrow. To maintain the blowing agent at a sufficient thickness, the viscosity of the blowing agent can be increased. However, in this case, a large stress is applied to the bonding wires. This stress can harm the loop of the bonding wires, and sometimes causes a contact between the chip and the bonding wires. Furthermore, it is difficult to spread the blowing agent at a uniform thickness over the chip due to the high viscosity. Thus, it is difficult to get a desirable hollow portion. When the hollow portion is insufficient, and the inner wall contacts the chip, stress caused by contraction due to the heat cycle is applied to the chip during a reliability test. This causes a deterioration of the chip. The same problem occurs not only with the surface acoustic wave device, but also with other chips, such as semiconductor devices.